INT 21 - Phar Lap 386/DOS-Extender VMM - LIMIT PROGRAM'S EXTENDED MEMORY USAGE
        AX = 2521h
        EBX = max 4k pages of physical extended memory which program may use
Return: CF clear if successful
           EBX = maximum limit in pages
           ECX = minimum limit in pages
        CF set on error
            EAX = error code
                08h insufficient memory or -nopage switch used